The synthesis of urea is the largest net synthesis in the mammalian liver, where it consumes 15-50% of the ATP produced. With such a high priority given to this pathway it seems reasonable that it serves a crucial metabolic function. The actual function of ureagenesis may be two-fold: (1) to maintain pH homeostasis, and (2) to maintain ammonium homeostasis. The physiological significance of these two possible functions is important in the clinical detection and treatment of patients afflicted with urea cycle disorders, and to the general understanding of pH and ammonium homeostasis. The intent of this project is to understand the regulation of ureagenesis at the level of the enzyme activities and to elucidate the factors which modulate the synthesis of urea. One focus of this project will be to determine the sites at which either pH or ammonia controls ureagenesis and under what metabolic conditions these controls operate. The other major focus will be to investigate the formation of multi-enzyme complexes and/or metabolite channeling between urea cycle enzymes as a possible mechanism for metabolic organization and regulation.